EFFECTS OF STERIC AND ELECTRONIC INTERACTIONS ON PI,PI-OVERLAP IN LANTHANIDE PORPHYRIN SANDWICH COMPLEXES

Electrochemical, infrared, resonance Raman, and optical absorption data are reported for the lanthanide porphyrin sandwich complexes, CeIV(TPP)2, CeIV(TPP)2+, EuIII(TPP)2, CeIV(TPnP)2, CeIV(TPnP)2+, and EuIII(TPnP)2 (TPP = meso-tetraphenylporphyrin; TPnP = meso-tetrapentylporphyrin). The two EuIII complexes contain a single hole in the porphyrin π system and are electronically similar to the CeIV sandwich porphyrin cation radicals. Variable-temperature UV-vis and near-infrared (near-IR) spectra are obtained for all four single-hole sandwiches. At high resolution and/or at low temperatures, well-resolved fine structure is observed on the intradimer charge-transfer bands (ca. 1300 and 1100 nm for the TPP and TPnP complexes, respectively). These absorptions are dominated by a single Franck-Condon-active vibration. This vibration is assigned as a mode, QAB, which contains a significant amount of multicenter character and modulates inter-ring separation. Vibronic analysis of the near-IR band contours reveals that multiple system origins (2-3) are present. The ground- and excited-state frequencies of the QAB mode increase and the dimensionless origin shifts along this coordinate decrease as the frequency of the system origin increases. The bluer system origins and higher vibrational frequencies are indicative of larger ππ overlap and a stronger metal-porphyrin bond, respectively. In the case of the Ln(TPnP)2 complexes, both the relative contributions of the individual progressions to the total intensity of the near-IR band and the frequencies of the RR bands are concentration dependent. In contrast, no concentration dependence is observed for the near-IR and RR bands of the Ln(TPP)2 complexes. Collectively, the spectral data indicate that the extent of ππ overlap is influenced by both steric and electronic factors. The steric contributions are determined by the relative orientations of the phenyl or pentyl substituents. Multiple conformations of these groups are accessible both at room and at low temperatures. For certain conformers, steric effects can override electronic factors. As a consequence, certain conformers of the Ln(TPP)2 complexes exhibit equal or greater ππ overlap than certain conformers of the Ln(TPnP)2 complexes. © 1990 American Chemical Society.